This is a continuation-in-part of application Ser. No. 484,730, filed Feb. 23, 1990.
The present invention relates to passivating postrinsing of conversion layers on metals, particularly steel, galvanized steel, zinc alloy-plated steel and aluminum, with a chromium-free aqueous solution before the application of a paint or adhesive.
The application of conversion layers is industrially employed on a large scale for the preparation of metal surfaces for a subsequent application of paint. The conversion layers thus formed result, inter alia, in an improved adhesion of the paint films on the metals, an increased resistance to corrosion and an inhibition of subsurface corrosion which might be initiated at damaged portions of the paint film. The conversion layers are those known as phosphate layers, those formed by an alkaline solution which contains at least two different polyvalent metal ions and those formed using a solution based on titanium, zirconium and/or hafnium ions. The protective properties of the conversion layers may further be improved by a passivating postrinsing with an aqueous fluid.
Desirable properties from the aspect of application technology are obtained from a passivating postrinsing with fluids which contain hexavalent and/or trivalent chromium. However, toxicity of the trivalent and particularly of the hexavalent chromium compounds is often regarded as a disadvantage.
U.S. Pat. No. 4,376,000 describes a chromium-free postrinsing agent which contains a comparatively high concentration of polyvinylphenol. The use of this agent results in an undesired pollution of the sewage, particularly because a large amount of oxygen is required for the decomposition.
U.S. Pat. No. 3,695,942 discloses the use of soluble zirconium components for an aftertreatment of conversion layers. In addition to zirconium, the postrinsing agents contain cations consisting of alkali and ammonium. The reference contains an explicit warning against the use of alkaline earth metal cations. These postrinsing agents, which are used at a pH value from 3 to 8.5, do not result in the same quality as the chromium-containing agents.
U.S. Pat. No.3,895,970 describes acid aqueous postrinsing agents for treating phosphate layers. Such agents containing simple or complex fluorides and chromium-zirconium fluoride and zirconium fluoride are mentioned as zirconium compounds. With the exception of chromium-zirconium fluoride, the products mentioned in that patent will meet only medium requirements. Chromium-zirconium fluoride has the above-mentioned disadvantage that it is toxic.
A process for producing a conversion layer on a surface of zinc or zinc alloy followed in a subsequent stage by rinsing with a rinsing solution is known from DE-C-1 521 854 in particular as a pretreatment prior to a paint coating or film coating procedure.
In order to form a layer on a zinc or zinc alloy surface, the process known from DE-C-1 521 854 makes use of an aqueous alkaline solution which contains ions of one or more of the metals silver, magnesium, cadmium, aluminum, tin, titanium, antimony, molybdenum, chromium, cerium, tungsten, manganese, cobalt, iron and nickel as so-called non-alkaline metal ions. Solutions which contain ions of iron or cobalt with one or more of the listed metals as ions are considered to be particularly suitable. Moreover, the solutions contain organic complex former in a sufficient amount so as to maintain the non-alkaline metal ions in solution. The conversion layers formed by means of these ions have increased corrosion resistance and improved adherence of subsequently applied organic coatings. The metal surfaces on which the conversion layers are formed show improved corrosion resistance and adherence when rinsed with a solution containing an acid, hexavalent chromium and, optionally, additional trivalent chromium.
Although that process for producing a conversion layer on a zinc or zinc alloy surface provides good corrosion protection and paint adherence, the use of tri- and, in particular, hexavalent chromium ions in a passivating rinsing solution is disadvantageous due to the toxicity and the necessary special waste disposal of the hexavalent chromium (chromate detoxification).
Industrial production of conversion layers on aluminum or aluminum alloy surfaces, in particular as a pretreatment prior the application of an organic coating, also is practiced extensively. Conversion layers of this kind prevent or inhibit corrosion, and in case of an organic coating, provide an improved adherence of the applied coating. Well known chromating layers meet these requirements to a sufficient extent. However, for hygienic reasons at the work place, for reasons of environmental protection, and since the treated material is used for special purposes such as the packing of foods, the production of chromium-free conversion layers is increasingly preferred. The treatment solutions used for this purpose are generally adjusted to an acid pH value and contain, for example, titanium, fluoride, phosphate and tannin (U.S. Pat. No. 4,017,334) or zirconium, fluoride, and boron (U.S. Pat. No. 3,964,936). A treatment solution containing hafnium and fluoride is described in FR 2,417,537.